Fix docs building

We need graphviz for docs building, use "doc" tag in bindep for it.
This is needed with updated jobs that look for doc tags.

Change-Id: Idc329ab113761061678c9bd31485ac5e18273e23

bindep.txt

@@ -8,7 +8,7 @@ openssl [test]
 zookeeperd [platform:dpkg]
 build-essential [platform:dpkg]
 gcc [platform:rpm]
-graphviz [test]
+graphviz [doc]
 libssl-dev [platform:dpkg]
 openssl-devel [platform:rpm]
 libffi-dev [platform:dpkg]